Aqueous solutions of propylene glycol monomethyl ether have been suggested for general use as coolants for internal combustion engines and have been sold commercially for such use in heavy piston-type engines such as are used in buses and heavy trucks but have not been found commercially acceptable for use in automobile engines.
Wankel engines, herein referred to generically as rotary engines, have peculiar coolant problems in that the rotor housing is asymmetrically heated "hot spots" are formed on the combustion side and as a result in extreme cases, severe engine damage can occur. The danger arising from high metal temperatures is made more acute by the fact that as hot spots develop there is often a tendency for solid deposits to form on the coolant side, which deposits further reduce heat transfer, thus aggravating the problem. It is accordingly highly desirable to prevent even transient formation of hot spots under severe conditions, such as high speed, heavy load, rapid acceleration, etc.
While water is the most readily available and effective coolant, antifreeze additives are necessary for cold weather use. Ethylene glycol is by far the most widely used antifreeze additive. It is less efficient than water but more efficient than propylene glycol and similar materials.
There is increasing concern about the use of ethylene glycol because of its high toxicity. Propylene glycol and its ethers are much less toxic than the corresponding ethylene compounds.
In most areas glycol antifreeze is used at a concentration of about 50-60% by weight. However, because of the common habit of adding additional concentrate from time to time to assure that a safe concentration of both antifreeze and corrosion inhibitors is present, it is not unusual to find autos carry antifreeze concentractions of 70% or more. Since the heat transfer efficiency of the fluid decreases as the concentration of antifreeze additive increases, such high concentrations are conducive to the formation of hot spots where metal temperatures are beyond the safe range.
In view of the above problems, it is an object of the present invention to provide a coolant for rotary engines that adquately protects against freezing and also against excessively hot spots on the combustion chamber wall when operating the engine under severe conditions and which has low animal and human toxicity.